Experian Hitwise takes a stab at estimating the early growth of Google+. For the week that ended July 16, the Google+ domain (plus.google.com) was still tiny with 1.8 million total estimated visits. However, that number was up 283 percent from the previous week, and up 821 percent from the week before. On a daily basis, estimated visits peaked last Thursday on July 14 with 317,000 estimated daily visits. That number settled down to 226,000 by July 18th, the last day of the data set.

Before you jump to any conclusions about Google+ slowing down, note that this data ends the day before Google introduced the new Google+ iPhone app, which is currently the top downloaded free app in iTunes. I know that at least for me the mobile app drove me to use Google+ much more than before, even on the Web. (Hitwise’s data doesn’t take into account mobile apps or the mobile Web anyway). Also, a lot of the usage comes from Google’s top nav bar across its sites, AKA the Sandbar, which Hitwise doesn’t count.

Still, what can we learn from the Hitwise numbers? They suggest that despite the millions of registered users, only a small percentage are actually using the site. The 1.8 million is visits, not unique visitors, so you’d have to divide it t get to an estimate of actual users, which Hitwise does not provide. But it’s pretty safe to assume that it is less than 1 million. A few hundred thousand active users in a few weeks still isn’t bad, but it’s nowhere near the 10 million to 20 million registered user number. Anyone can register for a service. It doesn’t mean they will come back to use it.

Hitwise says Google+ is the No. 42 social networking site in the U.S., and No. 638 overall. The week before it was ranked No. 2,404 among all sites in the U.S., so it is moving up quickly.

The majority of traffic to Google+ (56 percent) is coming from other Google properties, with Google search accounting for a full third (34 percent). Another 21 percent comes from email (those email notifications seem to be working).

Hitwise also sheds some light on the gender breakdown of Google+ users, with 57 percent of them being male for the four weeks ended July 16th. Users are also getting older, with 38 percent being between 25 and 34 years old for the last week, edging out the 18 to 24 year olds who made up the largest user group the week before. The top cities for Google+ users are, in order, LA (MySpace refugees?), New York, and San Francisco.